The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmissions. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of personal communication networks (PCN) (employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make and receive calls in the home, office, street, car, etc.), would be provided by the cellular carriers using the next generation digital cellular system infrastructure and the cellular frequencies. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is achieved using frequency division multiple access (FDMA) and time division multiple access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signals transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel radios.
In TDMA systems, a channel consists of a time slot in a period train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster bit rate which increases the amount of occupied spectrum proportionally.
With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, CDMA allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other.
In principle, the information data stream to be transmitted is first coded or spread and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The informational data stream and the PN-sequence or the scrambling-sequence are combined by multiplying the two bit streams together. Each information data stream or channel is allocated a unique spreading code. A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise related signals, in both frequency and time. By correlating the composite signal with one of the unique codes, a corresponding information signal is isolated and decoded.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA based cellular systems are projected to be up to 20 times that of existing analog technology as a result of the properties of a wideband CDMA system, such as improved coding gain/modulation density, voice activating gating, sectorization and reuse of the same spectrum in every cell. CDMA transmission of voice by a high bit rate decoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA completely eliminates cross-talk and makes it very difficult and costly to eavesdrop or track calls, ensuring greater privacy to callers and greater immunity from air-time fraud.
Despite the numerous advantages offered by CDMA, problems can occur when the CDMA system contains different size cells which have different power levels. One problem is how to allocate frequencies between the different types of cells. While this problem can be easily handed in traditional FDMA or TDMA systems, the problem is quite serious in CDMA systems because all of the frequencies are used throughout the system. This problem occurs, for example, when microcells are used within umbrella cells and at the border between urban and rural areas where different size cells are used. The general problem is that the uplink (from mobile station to base station) and downlink (from base station to mobile station) handoff points are not located at the same place. The downlink handoff point is located closer to the microcell than the uplink handoff point.
FIG. 1 illustrates a typical scenario of the use of microcells within umbrella cells. An umbrella cell 10 contains a base station 12 and a plurality of microcells 14. Each microcell 14 contains a base station 16. In this example, a mobile station 18 is located near the umbrella base station 12 but is located in a microcell 14. The base station for an umbrella cell 10 generally operates at a power level which is much higher than the power level used for base stations of a microcell. Since the mobile station is located in the microcell 14 and is in communication with the base station 16, the high powered signals from the umbrella base station 12 may interfere with the communications between the mobile station 18 and the microcell base station 16. Since the umbrella base station is operating at a high power level, the interfering signals can easily be 10-20 decibels above the communication signal between the mobile station and the microcell base station. Even if the processing gain of the CDMA system is large enough to handle such interfering signals, the capacity of the system will be decreased. Furthermore, if the mobile station 18 were to connect to the umbrella cell base station 12, the mobile station 18 would have to increase its power which would interfere with the microcell base station 16 in the uplink direction.
FIG. 2 illustrates the problems that can occur around the border between small (urban) and large (rural) cells. A rural cell 20 contains a base station 22 and an urban cell 24 contains a base station 26. In this example, a mobile station is located near the border between the urban cell 24 and the rural cell 20. When the mobile station is moving in the direction of the arrow A, the question becomes to which cell does the mobile station belong. If the mobile station is connected to the base station 26 in the urban cell 24, the mobile station may encounter an interference from the signals from the rural base station 22 due to the different power levels between the rural and urban base stations. If the mobile station is connected to the base station 22 in the rural cell 20, the mobile station will have to increase the power of its own transmission in order to adequately communicate with the base station 22. As a result, the mobile station's transmission will interfere with the reception of the base station 26 since the mobile station is closer to that base station.